Network Working Group P. Leach
Request for Comments: 4122 Microsoft
Category: Standards Track M. Mealling
Refactored Networks, LLC
R. Salz
DataPower Technology, Inc.
July 2005
A Universally Unique IDentifier (UUID) URN Namespace
Status of This Memo
This document specifies an Internet standards track protocol for the
Internet community, and requests discussion and suggestions for
improvements. Please refer to the current edition of the "Internet
Official Protocol Standards" (STD 1) for the standardization state
and status of this protocol. Distribution of this memo is unlimited.
Copyright Notice
Copyright (C) The Internet Society (2005).
Abstract
This specification defines a Uniform Resource Name namespace for
UUIDs (Universally Unique IDentifier), also known as GUIDs (Globally
Unique IDentifier). A UUID is 128 bits long, and can guarantee
uniqueness across space and time. UUIDs were originally used in the
Apollo Network Computing System and later in the Open Software
Foundation's (OSF) Distributed Computing Environment (DCE), and then
in Microsoft Windows platforms.
This specification is derived from the DCE specification with the
kind permission of the OSF (now known as The Open Group).
Information from earlier versions of the DCE specification have been
incorporated into this document.
Leach, et al. Standards Track [Page 1]

RFC 4122 A UUID URN Namespace July 20052. Motivation
One of the main reasons for using UUIDs is that no centralized
authority is required to administer them (although one format uses
IEEE 802 node identifiers, others do not). As a result, generation
on demand can be completely automated, and used for a variety of
purposes. The UUID generation algorithm described here supports very
high allocation rates of up to 10 million per second per machine if
necessary, so that they could even be used as transaction IDs.
UUIDs are of a fixed size (128 bits) which is reasonably small
compared to other alternatives. This lends itself well to sorting,
ordering, and hashing of all sorts, storing in databases, simple
allocation, and ease of programming in general.
Since UUIDs are unique and persistent, they make excellent Uniform
Resource Names. The unique ability to generate a new UUID without a
registration process allows for UUIDs to be one of the URNs with the
lowest minting cost.
3. Namespace Registration Template
Namespace ID: UUID
Registration Information:
Registration date: 2003-10-01
Declared registrant of the namespace:
JTC 1/SC6 (ASN.1 Rapporteur Group)
Declaration of syntactic structure:
A UUID is an identifier that is unique across both space and time,
with respect to the space of all UUIDs. Since a UUID is a fixed
size and contains a time field, it is possible for values to
rollover (around A.D. 3400, depending on the specific algorithm
used). A UUID can be used for multiple purposes, from tagging
objects with an extremely short lifetime, to reliably identifying
very persistent objects across a network.
The internal representation of a UUID is a specific sequence of
bits in memory, as described in Section 4. To accurately
represent a UUID as a URN, it is necessary to convert the bit
sequence to a string representation.
Each field is treated as an integer and has its value printed as a
zero-filled hexadecimal digit string with the most significant
digit first. The hexadecimal values "a" through "f" are output as
lower case characters and are case insensitive on input.
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RFC 4122 A UUID URN Namespace July 2005
The formal definition of the UUID string representation is
provided by the following ABNF [7]:
UUID = time-low "-" time-mid "-"
time-high-and-version "-"
clock-seq-and-reserved
clock-seq-low "-" node
time-low = 4hexOctet
time-mid = 2hexOctet
time-high-and-version = 2hexOctet
clock-seq-and-reserved = hexOctet
clock-seq-low = hexOctet
node = 6hexOctet
hexOctet = hexDigit hexDigit
hexDigit =
"0" / "1" / "2" / "3" / "4" / "5" / "6" / "7" / "8" / "9" /
"a" / "b" / "c" / "d" / "e" / "f" /
"A" / "B" / "C" / "D" / "E" / "F"
The following is an example of the string representation of a UUID as
a URN:
urn:uuid:f81d4fae-7dec-11d0-a765-00a0c91e6bf6
Relevant ancillary documentation:
[1][2]
Identifier uniqueness considerations:
This document specifies three algorithms to generate UUIDs: the
first leverages the unique values of 802 MAC addresses to
guarantee uniqueness, the second uses pseudo-random number
generators, and the third uses cryptographic hashing and
application-provided text strings. As a result, the UUIDs
generated according to the mechanisms here will be unique from all
other UUIDs that have been or will be assigned.
Identifier persistence considerations:
UUIDs are inherently very difficult to resolve in a global sense.
This, coupled with the fact that UUIDs are temporally unique
within their spatial context, ensures that UUIDs will remain as
persistent as possible.
Process of identifier assignment:
Generating a UUID does not require that a registration authority
be contacted. One algorithm requires a unique value over space
for each generator. This value is typically an IEEE 802 MAC
address, usually already available on network-connected hosts.
The address can be assigned from an address block obtained from
the IEEE registration authority. If no such address is available,
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RFC 4122 A UUID URN Namespace July 2005
or privacy concerns make its use undesirable, Section 4.5
specifies two alternatives. Another approach is to use version 3
or version 4 UUIDs as defined below.
Process for identifier resolution:
Since UUIDs are not globally resolvable, this is not applicable.
Rules for Lexical Equivalence:
Consider each field of the UUID to be an unsigned integer as shown
in the table in section Section 4.1.2. Then, to compare a pair of
UUIDs, arithmetically compare the corresponding fields from each
UUID in order of significance and according to their data type.
Two UUIDs are equal if and only if all the corresponding fields
are equal.
As an implementation note, equality comparison can be performed on
many systems by doing the appropriate byte-order canonicalization,
and then treating the two UUIDs as 128-bit unsigned integers.
UUIDs, as defined in this document, can also be ordered
lexicographically. For a pair of UUIDs, the first one follows the
second if the most significant field in which the UUIDs differ is
greater for the first UUID. The second precedes the first if the
most significant field in which the UUIDs differ is greater for
the second UUID.
Conformance with URN Syntax:
The string representation of a UUID is fully compatible with the
URN syntax. When converting from a bit-oriented, in-memory
representation of a UUID into a URN, care must be taken to
strictly adhere to the byte order issues mentioned in the string
representation section.
Validation mechanism:
Apart from determining whether the timestamp portion of the UUID
is in the future and therefore not yet assignable, there is no
mechanism for determining whether a UUID is 'valid'.
Scope:
UUIDs are global in scope.
4. Specification4.1. Format
The UUID format is 16 octets; some bits of the eight octet variant
field specified below determine finer structure.
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RFC 4122 A UUID URN Namespace July 20054.1.1. Variant
The variant field determines the layout of the UUID. That is, the
interpretation of all other bits in the UUID depends on the setting
of the bits in the variant field. As such, it could more accurately
be called a type field; we retain the original term for
compatibility. The variant field consists of a variable number of
the most significant bits of octet 8 of the UUID.
The following table lists the contents of the variant field, where
the letter "x" indicates a "don't-care" value.
Msb0 Msb1 Msb2 Description
0 x x Reserved, NCS backward compatibility.
1 0 x The variant specified in this document.
1 1 0 Reserved, Microsoft Corporation backward
compatibility
1 1 1 Reserved for future definition.
Interoperability, in any form, with variants other than the one
defined here is not guaranteed, and is not likely to be an issue in
practice.
4.1.2. Layout and Byte Order
To minimize confusion about bit assignments within octets, the UUID
record definition is defined only in terms of fields that are
integral numbers of octets. The fields are presented with the most
significant one first.
Field Data Type Octet Note
#
time_low unsigned 32 0-3 The low field of the
bit integer timestamp
time_mid unsigned 16 4-5 The middle field of the
bit integer timestamp
time_hi_and_version unsigned 16 6-7 The high field of the
bit integer timestamp multiplexed
with the version number
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RFC 4122 A UUID URN Namespace July 2005
0 0 1 1 3 The name-based version
specified in this document
that uses MD5 hashing.
0 1 0 0 4 The randomly or pseudo-
randomly generated version
specified in this document.
0 1 0 1 5 The name-based version
specified in this document
that uses SHA-1 hashing.
The version is more accurately a sub-type; again, we retain the term
for compatibility.
4.1.4. Timestamp
The timestamp is a 60-bit value. For UUID version 1, this is
represented by Coordinated Universal Time (UTC) as a count of 100-
nanosecond intervals since 00:00:00.00, 15 October 1582 (the date of
Gregorian reform to the Christian calendar).
For systems that do not have UTC available, but do have the local
time, they may use that instead of UTC, as long as they do so
consistently throughout the system. However, this is not recommended
since generating the UTC from local time only needs a time zone
offset.
For UUID version 3 or 5, the timestamp is a 60-bit value constructed
from a name as described in Section 4.3.
For UUID version 4, the timestamp is a randomly or pseudo-randomly
generated 60-bit value, as described in Section 4.4.
4.1.5. Clock Sequence
For UUID version 1, the clock sequence is used to help avoid
duplicates that could arise when the clock is set backwards in time
or if the node ID changes.
If the clock is set backwards, or might have been set backwards
(e.g., while the system was powered off), and the UUID generator can
not be sure that no UUIDs were generated with timestamps larger than
the value to which the clock was set, then the clock sequence has to
be changed. If the previous value of the clock sequence is known, it
can just be incremented; otherwise it should be set to a random or
high-quality pseudo-random value.
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RFC 4122 A UUID URN Namespace July 2005
Similarly, if the node ID changes (e.g., because a network card has
been moved between machines), setting the clock sequence to a random
number minimizes the probability of a duplicate due to slight
differences in the clock settings of the machines. If the value of
clock sequence associated with the changed node ID were known, then
the clock sequence could just be incremented, but that is unlikely.
The clock sequence MUST be originally (i.e., once in the lifetime of
a system) initialized to a random number to minimize the correlation
across systems. This provides maximum protection against node
identifiers that may move or switch from system to system rapidly.
The initial value MUST NOT be correlated to the node identifier.
For UUID version 3 or 5, the clock sequence is a 14-bit value
constructed from a name as described in Section 4.3.
For UUID version 4, clock sequence is a randomly or pseudo-randomly
generated 14-bit value as described in Section 4.4.
4.1.6. Node
For UUID version 1, the node field consists of an IEEE 802 MAC
address, usually the host address. For systems with multiple IEEE
802 addresses, any available one can be used. The lowest addressed
octet (octet number 10) contains the global/local bit and the
unicast/multicast bit, and is the first octet of the address
transmitted on an 802.3 LAN.
For systems with no IEEE address, a randomly or pseudo-randomly
generated value may be used; see Section 4.5. The multicast bit must
be set in such addresses, in order that they will never conflict with
addresses obtained from network cards.
For UUID version 3 or 5, the node field is a 48-bit value constructed
from a name as described in Section 4.3.
For UUID version 4, the node field is a randomly or pseudo-randomly
generated 48-bit value as described in Section 4.4.
4.1.7. Nil UUID
The nil UUID is special form of UUID that is specified to have all
128 bits set to zero.
4.2. Algorithms for Creating a Time-Based UUID
Various aspects of the algorithm for creating a version 1 UUID are
discussed in the following sections.
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RFC 4122 A UUID URN Namespace July 20054.2.1. Basic Algorithm
The following algorithm is simple, correct, and inefficient:
o Obtain a system-wide global lock
o From a system-wide shared stable store (e.g., a file), read the
UUID generator state: the values of the timestamp, clock sequence,
and node ID used to generate the last UUID.
o Get the current time as a 60-bit count of 100-nanosecond intervals
since 00:00:00.00, 15 October 1582.
o Get the current node ID.
o If the state was unavailable (e.g., non-existent or corrupted), or
the saved node ID is different than the current node ID, generate
a random clock sequence value.
o If the state was available, but the saved timestamp is later than
the current timestamp, increment the clock sequence value.
o Save the state (current timestamp, clock sequence, and node ID)
back to the stable store.
o Release the global lock.
o Format a UUID from the current timestamp, clock sequence, and node
ID values according to the steps in Section 4.2.2.
If UUIDs do not need to be frequently generated, the above algorithm
may be perfectly adequate. For higher performance requirements,
however, issues with the basic algorithm include:
o Reading the state from stable storage each time is inefficient.
o The resolution of the system clock may not be 100-nanoseconds.
o Writing the state to stable storage each time is inefficient.
o Sharing the state across process boundaries may be inefficient.
Each of these issues can be addressed in a modular fashion by local
improvements in the functions that read and write the state and read
the clock. We address each of them in turn in the following
sections.
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RFC 4122 A UUID URN Namespace July 20054.2.1.1. Reading Stable Storage
The state only needs to be read from stable storage once at boot
time, if it is read into a system-wide shared volatile store (and
updated whenever the stable store is updated).
If an implementation does not have any stable store available, then
it can always say that the values were unavailable. This is the
least desirable implementation because it will increase the frequency
of creation of new clock sequence numbers, which increases the
probability of duplicates.
If the node ID can never change (e.g., the net card is inseparable
from the system), or if any change also reinitializes the clock
sequence to a random value, then instead of keeping it in stable
store, the current node ID may be returned.
4.2.1.2. System Clock Resolution
The timestamp is generated from the system time, whose resolution may
be less than the resolution of the UUID timestamp.
If UUIDs do not need to be frequently generated, the timestamp can
simply be the system time multiplied by the number of 100-nanosecond
intervals per system time interval.
If a system overruns the generator by requesting too many UUIDs
within a single system time interval, the UUID service MUST either
return an error, or stall the UUID generator until the system clock
catches up.
A high resolution timestamp can be simulated by keeping a count of
the number of UUIDs that have been generated with the same value of
the system time, and using it to construct the low order bits of the
timestamp. The count will range between zero and the number of
100-nanosecond intervals per system time interval.
Note: If the processors overrun the UUID generation frequently,
additional node identifiers can be allocated to the system, which
will permit higher speed allocation by making multiple UUIDs
potentially available for each time stamp value.
4.2.1.3. Writing Stable Storage
The state does not always need to be written to stable store every
time a UUID is generated. The timestamp in the stable store can be
periodically set to a value larger than any yet used in a UUID. As
long as the generated UUIDs have timestamps less than that value, and
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RFC 4122 A UUID URN Namespace July 2005
the clock sequence and node ID remain unchanged, only the shared
volatile copy of the state needs to be updated. Furthermore, if the
timestamp value in stable store is in the future by less than the
typical time it takes the system to reboot, a crash will not cause a
reinitialization of the clock sequence.
4.2.1.4. Sharing State Across Processes
If it is too expensive to access shared state each time a UUID is
generated, then the system-wide generator can be implemented to
allocate a block of time stamps each time it is called; a per-
process generator can allocate from that block until it is exhausted.
4.2.2. Generation Details
Version 1 UUIDs are generated according to the following algorithm:
o Determine the values for the UTC-based timestamp and clock
sequence to be used in the UUID, as described in Section 4.2.1.
o For the purposes of this algorithm, consider the timestamp to be a
60-bit unsigned integer and the clock sequence to be a 14-bit
unsigned integer. Sequentially number the bits in a field,
starting with zero for the least significant bit.
o Set the time_low field equal to the least significant 32 bits
(bits zero through 31) of the timestamp in the same order of
significance.
o Set the time_mid field equal to bits 32 through 47 from the
timestamp in the same order of significance.
o Set the 12 least significant bits (bits zero through 11) of the
time_hi_and_version field equal to bits 48 through 59 from the
timestamp in the same order of significance.
o Set the four most significant bits (bits 12 through 15) of the
time_hi_and_version field to the 4-bit version number
corresponding to the UUID version being created, as shown in the
table above.
o Set the clock_seq_low field to the eight least significant bits
(bits zero through 7) of the clock sequence in the same order of
significance.
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RFC 4122 A UUID URN Namespace July 2005
o Set the 6 least significant bits (bits zero through 5) of the
clock_seq_hi_and_reserved field to the 6 most significant bits
(bits 8 through 13) of the clock sequence in the same order of
significance.
o Set the two most significant bits (bits 6 and 7) of the
clock_seq_hi_and_reserved to zero and one, respectively.
o Set the node field to the 48-bit IEEE address in the same order of
significance as the address.
4.3. Algorithm for Creating a Name-Based UUID
The version 3 or 5 UUID is meant for generating UUIDs from "names"
that are drawn from, and unique within, some "name space". The
concept of name and name space should be broadly construed, and not
limited to textual names. For example, some name spaces are the
domain name system, URLs, ISO Object IDs (OIDs), X.500 Distinguished
Names (DNs), and reserved words in a programming language. The
mechanisms or conventions used for allocating names and ensuring
their uniqueness within their name spaces are beyond the scope of
this specification.
The requirements for these types of UUIDs are as follows:
o The UUIDs generated at different times from the same name in the
same namespace MUST be equal.
o The UUIDs generated from two different names in the same namespace
should be different (with very high probability).
o The UUIDs generated from the same name in two different namespaces
should be different with (very high probability).
o If two UUIDs that were generated from names are equal, then they
were generated from the same name in the same namespace (with very
high probability).
The algorithm for generating a UUID from a name and a name space are
as follows:
o Allocate a UUID to use as a "name space ID" for all UUIDs
generated from names in that name space; see Appendix C for some
pre-defined values.
o Choose either MD5 [4] or SHA-1 [8] as the hash algorithm; If
backward compatibility is not an issue, SHA-1 is preferred.
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RFC 4122 A UUID URN Namespace July 2005
o Convert the name to a canonical sequence of octets (as defined by
the standards or conventions of its name space); put the name
space ID in network byte order.
o Compute the hash of the name space ID concatenated with the name.
o Set octets zero through 3 of the time_low field to octets zero
through 3 of the hash.
o Set octets zero and one of the time_mid field to octets 4 and 5 of
the hash.
o Set octets zero and one of the time_hi_and_version field to octets
6 and 7 of the hash.
o Set the four most significant bits (bits 12 through 15) of the
time_hi_and_version field to the appropriate 4-bit version number
from Section 4.1.3.
o Set the clock_seq_hi_and_reserved field to octet 8 of the hash.
o Set the two most significant bits (bits 6 and 7) of the
clock_seq_hi_and_reserved to zero and one, respectively.
o Set the clock_seq_low field to octet 9 of the hash.
o Set octets zero through five of the node field to octets 10
through 15 of the hash.
o Convert the resulting UUID to local byte order.
4.4. Algorithms for Creating a UUID from Truly Random or Pseudo-Random Numbers
The version 4 UUID is meant for generating UUIDs from truly-random or
pseudo-random numbers.
The algorithm is as follows:
o Set the two most significant bits (bits 6 and 7) of the
clock_seq_hi_and_reserved to zero and one, respectively.
o Set the four most significant bits (bits 12 through 15) of the
time_hi_and_version field to the 4-bit version number from
Section 4.1.3.
o Set all the other bits to randomly (or pseudo-randomly) chosen
values.
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RFC 4122 A UUID URN Namespace July 2005
See Section 4.5 for a discussion on random numbers.
4.5. Node IDs that Do Not Identify the Host
This section describes how to generate a version 1 UUID if an IEEE
802 address is not available, or its use is not desired.
One approach is to contact the IEEE and get a separate block of
addresses. At the time of writing, the application could be found at
<http://standards.ieee.org/regauth/oui/pilot-ind.html>, and the cost
was US$550.
A better solution is to obtain a 47-bit cryptographic quality random
number and use it as the low 47 bits of the node ID, with the least
significant bit of the first octet of the node ID set to one. This
bit is the unicast/multicast bit, which will never be set in IEEE 802
addresses obtained from network cards. Hence, there can never be a
conflict between UUIDs generated by machines with and without network
cards. (Recall that the IEEE 802 spec talks about transmission
order, which is the opposite of the in-memory representation that is
discussed in this document.)
For compatibility with earlier specifications, note that this
document uses the unicast/multicast bit, instead of the arguably more
correct local/global bit.
Advice on generating cryptographic-quality random numbers can be
found in RFC1750 [5].
In addition, items such as the computer's name and the name of the
operating system, while not strictly speaking random, will help
differentiate the results from those obtained by other systems.
The exact algorithm to generate a node ID using these data is system
specific, because both the data available and the functions to obtain
them are often very system specific. A generic approach, however, is
to accumulate as many sources as possible into a buffer, use a
message digest such as MD5 [4] or SHA-1 [8], take an arbitrary 6
bytes from the hash value, and set the multicast bit as described
above.
5. Community Considerations
The use of UUIDs is extremely pervasive in computing. They comprise
the core identifier infrastructure for many operating systems
(Microsoft Windows) and applications (the Mozilla browser) and in
many cases, become exposed to the Web in many non-standard ways.
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RFC 4122 A UUID URN Namespace July 2005
This specification attempts to standardize that practice as openly as
possible and in a way that attempts to benefit the entire Internet.
6. Security Considerations
Do not assume that UUIDs are hard to guess; they should not be used
as security capabilities (identifiers whose mere possession grants
access), for example. A predictable random number source will
exacerbate the situation.
Do not assume that it is easy to determine if a UUID has been
slightly transposed in order to redirect a reference to another
object. Humans do not have the ability to easily check the integrity
of a UUID by simply glancing at it.
Distributed applications generating UUIDs at a variety of hosts must
be willing to rely on the random number source at all hosts. If this
is not feasible, the namespace variant should be used.
7. Acknowledgments
This document draws heavily on the OSF DCE specification for UUIDs.
Ted Ts'o provided helpful comments, especially on the byte ordering
section which we mostly plagiarized from a proposed wording he
supplied (all errors in that section are our responsibility,
however).
We are also grateful to the careful reading and bit-twiddling of Ralf
S. Engelschall, John Larmouth, and Paul Thorpe. Professor Larmouth
was also invaluable in achieving coordination with ISO/IEC.
8. Normative References
[1] Zahn, L., Dineen, T., and P. Leach, "Network Computing
Architecture", ISBN 0-13-611674-4, January 1990.
[2] "DCE: Remote Procedure Call", Open Group CAE Specification C309,
ISBN 1-85912-041-5, August 1994.
[3] ISO/IEC 9834-8:2004 Information Technology, "Procedures for the
operation of OSI Registration Authorities: Generation and
registration of Universally Unique Identifiers (UUIDs) and their
use as ASN.1 Object Identifier components" ITU-T Rec. X.667,
2004.
[4] Rivest, R., "The MD5 Message-Digest Algorithm ", RFC 1321, April
1992.
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RFC 4122 A UUID URN Namespace July 2005
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Leach, et al. Standards Track [Page 32]